Jeremy Corbyn is conducting the long-awaited reshuffle of his top team, amid intense speculation about who could take the shadow foreign and defence jobs.

The Labour leader confirmed on Monday that there would be a shakeup as he appeared at King’s Cross station in London for a campaign against rising rail fares.

He was expected to conduct the reshuffle from his office at Westminster, with some one-on-one meetings and phone conversations.

Most MPs are not yet back in Westminster after the Christmas break, but John McDonnell, the shadow chancellor, was seen heading towards Corbyn’s office on Monday afternoon, as was Seema Malhotra, the shadow chief secretary to the Treasury. Rosie Winterton, the chief whip who helped orchestrate Corbyn’s first appointments in September, entered the leader’s office at 2.30pm.

Corbyn’s most likely move is to replace Hilary Benn, the shadow foreign secretary, who opposed him over the Syria airstrikes vote, and Maria Eagle, the shadow defence secretary, who takes a different position from him on replacing the Trident nuclear submarines.

Ken Livingstone, an ally of Corbyn and former London mayor, told the BBC’s World at One that it “might very well be the case that it would be better” if Benn were to be moved to an area where his thinking was more in tune with that of the party leader.

Livingstone said he believed it was “quite likely” that Benn would be asked to swap jobs with the shadow home secretary, Andy Burnham, as speculation has suggested, though he insisted he did not know whether this was what Corbyn was planning.

However, some of the candidates mooted for promotion to foreign affairs and defence – Diane Abbott and Clive Lewis – have all but ruled out accepting the new roles.

Abbott, who had been tipped as a possible shadow foreign secretary, said: “It’s completely untrue. I’ve never been offered the job of [shadow] foreign secretary. There was never any question of me being offered it, or of it being debated … It’s just, as they say, poppycock and piffle.”

Lewis, a new MP who has served in the army in Afghanistan, said he would not be keen to take on a shadow cabinet role so quickly. He told the BBC: “I have been an MP for eight months and I’m already on the shadow frontbench.

“It’s a steep learning curve, being an MP; an even steeper learning curve being on the shadow frontbench. To be on the cabinet, it’s another league altogether. So I think I want to pace myself, like most people who come into parliament.”

He added: “If Jeremy Corbyn offered me that and said, ‘I need this, I need you to do it, this is essential that you do it,’ I would have to think about it ... But to be quite frank, no. My initial inclination would be to say no.”

The prospect of a reshuffle has triggered alarm among some of the more centrist members of the shadow cabinet. Pat McFadden, the shadow Europe minister, warned Corbyn not to conduct a “punishment” reshuffle to remove colleagues who had publicly disagreed with him, saying it could look petty and divisive.

He told the BBC’s Westminster Hour: “Every leader has a right to change their team … but it does strike me it has been only three months since Jeremy Corbyn appointed his shadow cabinet. So you have to ask, why would you have a reshuffle?

“If it’s about political disagreement, I think you have to pause here, especially if it’s about the Syria vote,” said McFadden.

He said it was possible that Labour aides had been exaggerating the idea that there would be a “revenge” reshuffle against dissenters, but added: “There is a danger for him in carrying out a reshuffle as a sort of punishment for shadow ministers who disagreed with him.

“[Corbyn] has talked of an open and pluralist politics, but a reshuffle for that reason could end up looking more petty and divisive than open and pluralist. That is a risk for him if he proceeds for that reason.”

Eleven of the 28 members of the shadow cabinet, including Benn and Eagle, voted in favour of extending airstrikes against Islamic State to Syria, a move Corbyn strongly opposed. Eagle also supports the renewal of Trident – something on which the Labour leader is keen to change the party’s position – which will be voted on in parliament in the spring.

The shadow culture secretary, Michael Dugher, one of those believed to be at risk in the impending reorganisation, told BBC Radio 5 Live’s Pienaar’s Politics programme on Sunday that previous Labour leaders with whom he had worked closely had been reluctant to “go down the path of big reshuffles”.

“They do try to hold the party together, they do recognise that the Labour party is a broad church, not a religious cult, that you need people of different backgrounds and try and get the best possible talents,” Dugher said, adding that “ultimately [the makeup of the shadow cabinet] will be a decision for Jeremy”.

The reshuffle risks deepening rifts in the party and some Labour MPs have been keen to point out that Corbyn began his leadership promising to allow members of his shadow cabinet to express differing views.

Labour sources have suggested the possibility that Burnham could swap positions with Benn, leaving both of them more likely to agree with the leadership on their respective briefs.

It has also been suggested that Benn could be replaced by Emily Thornberry, the MP for Islington South and Finsbury, the neighbouring constituency to Corbyn’s.